This application claims the priority benefit of Taiwan application serial no. 89119718, filed Sep. 25, 2000.
1. Field of Invention
The present invention relates to a chemical-mechanical polishing method. More particularly, the present invention relates to a method for changing the polishing selectivity ratio of slurry used in chemical-mechanical polishing.
2. Description of Related Art
As soon as semiconductor production enters the deep sub-micron and copper conductive wire era, dual damascene process will become an indispensable part of the fabrication process. Chemical-mechanical polishing of copper is a closely related technique in the dual damascene process.
To produce sophisticated integrated circuits with sufficient interconnecting wires on a limited chip surface, most silicon chip contains a multiple of metallic layers. In the conventional method, a multi-layered interconnect is formed by forming a via opening in a dielectric layer and then filling the opening with conductive material to form a via plug. Thereafter, a metallic layer is formed over the via plug. Conventional photolithographic and etching techniques are used to pattern out a metal wire. However, when the metallic layer is patterned by etching, some defects are likely produced in addition to the difficulties in dry etching a metal. The dual damascene process is a feasible substitute for the conventional fabrication process because the etching of dielectric material is much easier. A dual damascene process is formed by first etching a dielectric layer to form a via opening that connects with an underlying metallic line. A shallow trench is also formed in the dielectric layer serving as the location for forming an upper metallic line. Copper is deposited into the via opening and the shallow trench and then chemical-mechanical polishing is conducted to remove excess metal and planarize the copper layer.
In conventional dual damascene processes, important factors that have an effect on the chemical-mechanical polishing of copper include temperature, pH value, pressure applied on the chip during polishing, rotational speed of the polishing pad and material constituting the polishing pad. However, the most important factor is the constituents of slurry. The constituents of the slurry can directly affect the polishing selectivity of a particular polishing operation. Currently, development of chemical-mechanical polishing is heading towards improving planarity, increasing polishing rate, increasing etching selectivity between different polishing materials and improving the accuracy of end-point detection.
In general, chemical-mechanical polishing of a copper layer is carried out in two stages. A copper slurry is used in the first stage to remove excess metal above the barrier layer but polishing is stopped above the barrier layer, just before the barrier layer is completely removed. Barrier slurry is used in the second stage to remove any residual barrier layer but stopping above the dielectric layer. In the second stage, the effect of slurry on polishing selectivity between copper film, the barrier layer and the dielectric layer is most critical. In the conventional process, differences in stress between various materials and difference in polishing selectivity of slurry often lead to dishing of the metallic layer. This has the effect of reducing overall cross-sectional area of metallic lines and increasing metallic line resistance.
The following is a description of a two-stage chemical-mechanical polishing in a conventional dual damascene process. FIGS. 1 through 3 are schematic cross-sectional views showing the steps for producing a conventional dual damascene structure. As shown in FIG. 1, a substrate 100 having a dielectric layer 102 thereon is provided. The dielectric layer 102 can be a silicon dioxide layer or other low dielectric constant material. A via opening that exposes a portion of the substrate 100 is formed in the dielectric layer 102. A shallow trench is formed over the via opening for forming a metallic line. The via opening together with the shallow trench form a dual damascene opening 103. A barrier layer 104 conformal to the dual damascene opening 103 is formed over the exposed surface of the substrate 100. The barrier layer 104 can be a titanium/titanium nitride or a tantalum/tantalum nitride layer, for example. Finally, a metallic layer 106 is formed over the barrier layer 104.
As shown in FIG. 2, copper slurry is used in a first stage polishing operation to remove excess metal from the metallic layer 106 outside the opening 103 and above the barrier layer 104. The polishing operation stops before the barrier layer 104 is completely removed. The main constituent of the copper slurry can be a neutral substance such as aluminum oxide (Al2O3), but the main constituent of the copper slurry can also be an acid substance such as silicon dioxide (SiO2).
As shown in FIG. 3, barrier slurry is used in a second stage polishing operation to remove residual barrier layer 104. The polishing operation stops above the dielectric layer 102. The barrier slurry can be an acid or a basic silicon dioxide solution. In brief, a conventional chemical-mechanical polishing involves removing excess copper in the copper layer 106 using a copper slurry and then removing the barrier layer 104 using a barrier slurry. Since both the copper slurry and the barrier slurry have a fixed polishing rate relative to various substances, dishing of the metallic layer 106 frequently occurs after polishing.
Accordingly, one object of the present invention is to provide a method for changing polishing selectivity ratio of slurry in a chemical-mechanical polishing process.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method for changing polishing selectivity ratio of slurry. The invention involves mixing barrier slurry with a diluent at different ratios so that a variety of different polishing selectivities are obtained. The diluent is capable of maintaining the pH value of the original slurry. The mixture is transported to the polishing pad of a polishing station to carry out polishing. The diluent is a buffer solution capable of maintaining a constant pH value. The diluent, for example, can be a solution of de-ionized water with benzotriazole (BTA, C6H5N3) or a solution of de-ionized water with BTA and polyacrylic acid (PAA).
According to the invention, slurry and diluent are mixed in different ratios to modify the relative content of solvents, chemicals and slurry particles within the barrier slurry. Hence, different polishing selectivity ratios are obtained between the copper film, the barrier layer and the dielectric layer (including low dielectric constant material). By changing the polishing selectivity ratio through mixing constituents in different amount, dishing of the metallic layer is prevented. The diluent and the slurry can be transported to the polishing pad through separate pipelines. Alternatively, the diluent and the slurry are separately pumped to the same pipeline and mixed before being delivered to the polishing pad for polishing.
The slurry used in chemical-mechanical polishing is modified in-situ so that polishing selectivity ratio between different material layers can be adjusted to obtain the optimal polishing results.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.